This invention relates to radar systems and more particularly to a radar system and components therefore for transmitting an electromagnetic signal from above the surface of water to below the surface of water to detect anomalies below the surface of water.
As is known in the art, it is often desirable to detect the present of objects under the surface of water. Typically, conventional underwater detection systems operate on acoustic principles. That is, acoustic energy is transmitted in the water medium and echo return signals are received and processed to determine the presence of objects. However, acoustic signal propagation requires a transmitting transducer and a receiving transducer be disposed in the water medium. If an underwater detection system is utilized from an aircraft (i.e. airplane, helicopter, etc.), then the transmitting transducer and the receiving transducer must be suspended from a cable or a towline attached to the aircraft with the transmitting transducer and the receiving transducer disposed below the water's surface. Unfortunately, such an arrangement reduces the mobility of the aircraft and the rate of searching capability of the system. Thus, it is desirable to not require transducers disposed in the water for an underwater detection system utilized from an aircraft.
An alternative technique for detecting objects is utilizing a radar system transmitting electromagnetic signals. Electromagnetic signals propagate effectively in air and couple suitably to sea water. Unfortunately, electromagnetic signals do not propagate well in a water medium wherein the signals are subjected to a high rate of attenuation. In sea water, an electromagnetic signal is typically attenuated 8.68 dB per skin depth. Skin depth is frequency dependent and typically with a frequency of 1 Hz, the skin depth is 252 meters, with a frequency of 100 Hz, the skin depth is 25.2 meters and with a frequency of 10 KHz, the skin depth is only 2.52 meters. If a signal having a frequency of 100 Hz is propagating through sea water a distance of 1,000 meters, then the signal would have travelled a distance of approximately 40 skin depths which equals 347 dB of attenuation. From the latter, it should be appreciated that it is desirable to use relatively low frequency electromagnetic signals if the signal is required to penetrate the surface of the water a significant amount of distance.
An electric antenna for transmitting an electromagnetic signal is typically a multiple of an one-half wavelength or variant thereof such that the electromagnetic signal is resonant with the antenna. At frequencies approaching 100 Hz, a signal has a wavelength in air so long that an electric antenna is not practical for use in an aircraft. An alternative antenna for a signal with a frequency of 100 Hz, is a magnetic dipole antenna for transmitting the electromagnetic signal. One such antenna is described in a publication entitled "Air/Undersea Communication at Ultra-Low-Frequencies Using Airborne Loop Antennas" by A. C. Fraser-Smith, D. M. Bubenik and O. G. Villard, Jr. wherein a loop antenna is described for use in an airplane. However, such an antenna is large and spanned the entire aircraft. Also the antenna had a large inductance, which may not be a problem in some instances, but is typically not desirable.